The answer to this can be found in Chapter 10. There, Diamond tells us that crops can diffuse more easily across east-west axes. This means that it is much easier for civilizations to spring up along such axes. These societies are more likely to become powerful because there are so many of them.

Places along the same latitude tend to be similar in climate and other factors. This means that plants can diffuse along these lines. When one society becomes agricultural, others can copy from them. This means that there will be many civilizations springing up in relatively close proximity to one another. These civilizations can then exchange technologies and become much more powerful than civilizations that are isolated on north-south axes.